This invention relates to interference elimination in a mobile communications system, especially in systems using code division multiple access (CDMA) technology and/or multi-user detection.
The function of CDMA (Code Division Multiple Access) in radio systems is based on spread spectrum communication. The signal to be sent is hashed with an individual hash code assigned to the subscriber, whereby the transmission will spread out on a broad-band radio channel, which is e.g. 1.25; 6.4 or 20 MHz. It is hereby possible for several subscribers on the same broad-band radio channel at the same time to send CDMA signals processed with different hash codes. In CDMA systems, the special hash code of each subscriber will hereby produce a traffic channel in the system in the same manner as a time slot in TDMA systems. At the receiving end the CDMA signal is decoded with the subscriber""s hash code, whereby a narrow-band signal will result. Broad-band signals of other subscribers represent noise in the receiver beside the desired signal.
FIG. 1 in the attached drawing shows a block diagram of a typical mobile communications system. A mobile station MS is connected over a radio path to at least one base station BS, in the case shown in FIG. 1 to base station BS1. A base station controller BSC controls the function of all base stations BS under its control. Several base station controllers BSC are usually subordinated to a mobile services switching centre MSC. The mobile services switching centre MSC is connected to other mobile services switching centres and to other networks, e.g. to a public services telephone network PSTN, to another mobile station network PLMN, to an ISDN network ISDN or to a packet switched public data network PSPDN.
The mobile station MS is usually connected to a base station providing the best signal quality. During handover in the course of a call the mobile station can in CDMA systems be simultaneously connected to several base stations BS, until some base station signal turns out to be better than the others, whereby the call will continue through this base station BS. Such handover is called soft handover. The mobile station MS sends to the base station BS a coded, interleaved, hashed and modulated signal over the radio path. The base station BS receives a broad-band radio signal which includes the signals of several mobile stations MS and from which the individual signals must be detected. To powerful signals, a broad-band interference of other signals represents background noise only, but weak signals may be covered under the interference level caused by other signals, whereby it is difficult to detect them. This situation is illustrated in FIG. 2 by power levels of signals S1-S5 shown as a function of time. The power level of signal S1 in the figure is considerably above the power levels of the other signals S2-S5 and it hereby causes most interference to signals S2-S5.
Different common detection methods are known from the CDMA context for improving the detection of every individual subscriber""s signal by utilising the signal parameters of other subscribers. By combining signals arriving from surrounding cells in the common detection process it is possible in some cases further to improve the advantage which can be achieved with the method. Interference cancellation is generally performed by eliminating interference signals from the signals received at the base station so that the other signals are eliminated as serial or parallel processing from the received signal, whereby the interference level will be lowered from the viewpoint of the undetected signal. Interference cancellation may be performed with either a broad-band or a narrow-band signal. Interference cancellation is typically used to reduce interference in the same cell.
It is known at the base station to use so-called adaptive antennas, that is, alternating directional pattern antennas, with which the reception at the base station is directed to a narrow geographical area. Signals arriving from outside the main beam of the adaptive directional antenna will be attenuated compared with signals received from the main beam at a ratio determined by the directional pattern of the antenna. When using adaptive antennas, the chosen mobile station may send its signal at a transmission power which is lower than the normal power, thanks to the better antenna amplification and signal processing at the reception. Therefore, adaptive antennas are used mainly for reducing the interference caused in the neighbouring cell and for increasing the coverage area of the base station in the direction of the main beam of the antenna. The directional antennas may be adaptive ones, the direction of which can be changed, or they may be fixedly directed. The signal sent by the individual mobile station can be received through several antenna beams, whereby it is possible e.g. to combine signal components caused by multipath propagation or otherwise to process a mobile station signal received with different antennas, e.g. by weighting received signals in different ways so that the signal will be amplified compared with other received signals. The directional pattern of an adaptive antenna can thus be changed by receiving the signal through several fixedly directed antenna beams and by weighting the thus received signals in a suitable manner. With adaptive antennas a lowering of the interference level is achieved owing to the narrow antenna beam, as the interference from other subscribers is reduced and the interference caused to other subscribers is reduced.
The use of adaptive antennas is illustrated in FIG. 3, wherein cell coverage C1 is arranged for base station BS with an omnidirectional antenna provided with a fixed antenna pattern while cell coverages C2-C4 are arranged with directed antennas. In the case shown in FIG. 3, reception of the signal of mobile station MS1 is arranged at base station BS with the antenna of cell C2 whereas reception of the signal of mobile station MS3 is arranged with the antenna of cell C3. The signal of mobile station MS1 can also be received e.g. through the antenna of cell C3 and correspondingly the signal of mobile station MS3 can be received with the antenna of cell C2 and/or cell C4. The signals of mobile station MS1 which are received through different antennas are combined in order to facilitate detection of the signal. Likewise, all signals received from mobile station MS3 are combined. The signals of the other mobile stations MS2 and MS4 shown in the FIGURE are received with the omnidirectional antenna of cell C1. It is a drawback in the use of adaptive antennas that reception directed to all subscribers requires much processing in the network.
Patent application publication EP-491 668 presents a method of demodulation of a CDMA signal based on a reduction of interference which is performed in a signal strength order. The system presented in the publication includes arranging means with which the signals are arranged in an order according to their relative signal strength. In the order established by the arranging means each signal is demodulated, detected, re-modulated and subtracted from the common signal. Such interference cancellation requires much processing capacity of the system, when each signal is subtracted in turn from the common signal.
Patent application publication EP-493 904 presents an arrangement for detecting received CDMA signals from a set of several signals. In the arrangement a signal received at the base station is input to several reception branches, each of which will detect the signal of a certain traffic channel. In the reception branches the signal is demodulated, despread and each individual signal is detected. These individual signals are classified in regard to their information contents as probably correct and probably incorrect signals respectively. The signals which belong to the first class and the information of which is correct, are modulated and re-hashed, whereupon these broad-band signals are subtracted from the original received signal, whereby the interference level of the remaining signal is lowered. From the signal thus processed those signals are re-detected which were classified originally as containing incorrect information, in order to ensure correct information in all individual signals. It is a particular problem with the interference cancellation method presented in the publication that the reliability check, wherein signals are classified as probably correct and probably incorrect signals, will cause a delay in signal processing. This delay in signal processing is especially problematic for such signals for which a short delay only can be accepted in their transfer.
It is also a problem with the known interference elimination methods presented above that they are very complex, they demand much computing and thus require a great processing ability of the mobile communications system, that is, they consume the limited capacity of the system. Especially as the number of mobile station subscribers is growing, the need for processing capacity increases considerably, because the capacity must be dimensioned according to the maximum number of users.
It is a purpose of this invention to reduce interference in a mobile communications network by controlling the processing performed at the receiving end.
This objective is achieved with the method according to the invention which is characterised by what is stated in the independent claims 1, 14 and 28. Special embodiments of the invention are presented in the dependent claims.
The invention is based on the idea of classifying mobile stations, according to the interference caused by their signal to other signals or according to the interference tolerance of their signal or the complexity of decoding of their signal, into classes demanding different further processing, and of defining for each class a class-specific way of functioning, whereby the total network interference will be reduced by following these ways of functioning. The method according to the invention reduces the interference caused by those signals only whose interference reduction will result the biggest reduction of the overall interference caused to the signals of other subscribers, and/or which cause interference that can be most easily reduced. The interference caused by these chosen signals is reduced e.g. by directing directional beams of adaptive antennas at these mobile stations and/or by eliminating the most strongly interfering signals from the received signal and/or by establishing a powerful correction of errors for a certain set of signals. In the method according to the invention such a reduction of interference is performed on the signals of one class which will result in a simplified processing of the signals of another class.
The complexity of processing to be performed for reducing interference depends on the number of subscribers and hash codes to be processed. The computing needed for eliminating interference is simplified when according to the present invention the effect of the worst interference only is eliminated for other subscribers. In addition, it is most easy to estimate the signals of the strongest interference and to eliminate them from the other signals. The greatest advantage is also achieved by eliminating the strongest interference, as the relatively biggest part of the interference is eliminated.
It is indeed an advantage of the interference reduction according to the invention that a great benefit is achieved through interference elimination by using as little equipment resources of the mobile communications system as possible.
It is another advantage of such a method that it simplifies the interference elimination performed in the receiver.
A further advantage of the method according to the invention is that signal processing in the mobile communications system will not be jammed and that the processing capacity of the equipment in the system can be used for some other purpose.